Preparation magnetic graphene oxide/diethylenetriamine composite for removal of methylene blue from aqueous solutions

Graphene oxide (GO) and its derivatives have several applications in many areas such as environmental and energy materials, water treatment and biomedical technologies. Because of having various polar groups on its surface, GO is considered as an excellent adsorbent. However, for many applications such as adsorption of pollution from aqueous solutions, chemical functionalization of graphene oxide is often a necessary requirement. In the present study, a new composite from graphene oxide, diethylenetriamine (DETA) and silica coated MnFe2O4 nanoparticles (GO/DETA/MnFe2O4@SiO2) was prepared. The structure, thermal stability and magnetic properties of the composite were studied by FT-IR, XRD, SEM, EDS, VSM and TGA spectroscopic methods. The prepared composite showed magnetic property with a saturation magnetization of 3.0 emu/g. The adsorption properties of GO/DETA/MnFe2O4@SiO2 composite for methylene blue (MB) in aqueous solution were studied using batch method. The effects of important parameters on the surface adsorption process of MB, including pH, contact time, adsorbent dosage and initial dye concentration were investigated. The adsorption isotherm was in accordance with Langmuir model showing surface homogeneity of the adsorbent. According to the Langmuir analysis, the maximum adsorption capacity (qm) of GO/DETA/MnFe2O4@SiO2 composite for MB was found to be 243.91 mg/g. The kinetic studies showed that the adsorption was pseudo first-order process. In addition, the thermodynamic studies indicated the adsorption was spontaneous and endothermic process.

In this field, Graphene based nanocomposites have also been investigated for the removal of some organic dyes and MB from wastewater 25 .Graphene oxide (GO) has a special structure of a two-dimensional honeycomb lattice with a single layer of sp 2 carbon atoms 26,27 .Studies show that GO is an effective adsorbent towards dyes and heavy metal ions [28][29][30][31] .Its electronic and characteristic structure supply the electrostatic force and π-π stacking effect, which facilitate and contribute to the adsorption process [32][33][34][35] .Furthermore, GO is composed of many functional groups such as carboxyl groups, hydroxyl and epoxide, which lead to a hydrophilic and negatively charged surface.GO can be simply functionalized applying different treatments to change the functional groups for the purpose of gaining the desired surface properties.GO derivatives especially those functionalized with heteroatom have shown better performance as compared to its counter parts of graphene oxide 36,37 .In this filed, GO-calcium/alginate nanocomposite was synthesized and used to adsorb MB by Li et al., and the adsorption capacity was 163.93 mg g −1 38 .Fan et al. synthesized a magnetic GO-chitosan nanocomposite and used it for removal of MB from aqueous solutions.The adsorption capacity was determined to be 95.31 mg g −1 39 .In another study, Dai et al. doped the synthetic polymers such as PVA (poly (vinyl alcohol)) with graphene oxide in order to increase the ability of adsorption of MB.The adsorption capacity of GO/PVA composite for MB removal was 127.5 mg g −1 40 .
In this study, a new composite from GO, diethylenetriamine (DETA) and silica coated MnFe 2 O 4 nanoparticles (GO/DETA/MnFe 2 O 4 @SiO 2 ) was synthesized (Fig. 1).The composite prepared was applied for the removal of the MB from aqueous solution.Moreover, the effects of various parameters such as pH, adsorbent dosage, initial dye concentration and contact time on adsorption behavior were studied.Adsorption isotherms, kinetics and thermodynamic studies have been reported to account for the nature of adsorption process.

Infrared spectral characterization
The FT-IR spectra of synthesized compounds are demonstrated in Fig. 2. In MnFe 2 O 4 FT-IR spectrum (Fig. 2a, blue line), a broad peak at 3428 cm −1 and a peak at 1627 cm −1 belong to stretching and bending vibration of hydroxyl (OH) groups, respectively 41 .Also, the peaks at 582 and 463 cm −1 are attributed to stretching vibration of Metal oxide (M-O) stretching vibrations of Mn-O and Fe-O, respectively 42 .The FT-IR spectrum of MnFe 2 O 4 @SiO 2 nanoparticles is showed in Fig. 2b (black line).As it can be observed, MnFe 2 O 4 nanoparticles after reaction with TEOS demonstrates a new peak at 1077 cm −1 which is assigned to stretching vibration of Si-O-Si. Figure 2c (green line) presents the FT-IR spectrum of GO.According to the GO FT-IR spectrum, a broad peak at 3600-2500 cm −1 belongs to stretching vibration of hydroxyl groups (OH) and adsorbed water molecules.Furthermore, the C=O stretching vibration of carboxylic acid group demonstrates a sharp peak at 1739 cm −1 .Moreover, the peak at 1620 cm −1 is attributed to the stretching vibration mode of C=C band and the peaks at 1228 and 1052 cm −1 are attributed to the C-O stretching of phenolic and epoxy groups 43 .The FT-IR spectrum of GO/DETA/MnFe 2 O 4 @SiO 2 composite is showed in Fig. 2d (red line).As it can be observed, after DETA and MnFe 2 O 4 @SiO 2 modification, the peak of GO at 1739 cm −1 disappears indicating the reduction of C=O of carboxylic acid group because of amine functionalization.Also, new peaks appear at 1516 and 1085 cm −1 which belong to N-H bending and Si-O-Si stretching, respectively.Besides, in the FT-IR spectrum of GO the peak at 1381 cm −1 is attributed to C-OH band and due to ammination process its intensity has been decreased 44 .

XRD analysis
X-ray diffraction of MnFe 2 O 4 nanoparticles, GO and GO/DETA/MnFe 2 O 4 @SiO 2 composite are demonstrated in Fig. 3.For the MnFe 2 O 4 nanoparticles, XRD patterns (Fig. 3a) show the typical peaks 2θ = 30.20°,35.45°, 43.25°, 53.50°, 57.15° and 63.40°, which are in agreement with the referenced data for MnFe 2 O 4 nanoparticles 45 .Figure 3b presents XRD patterns of GO.The strong diffraction peak at 2θ = 10.49°shows that interlayer spacing of GO based on the Bragg equation is 8.4 Å. Figure 3c shows XRD patterns of GO/DETA/MnFe 2 O 4 @SiO 2 composite.The XRD pattern of GO/DETA/MnFe 2 O 4 @SiO 2 composite includes the MnFe 2 O 4 peaks with a peak at 2θ = 9.8° which is attributed to GO.The average particles size of GO/DETA/MnFe 2 O 4 @SiO 2 composite is calculated via Debye-Scherer equation: where D is the average size, λ is the X-ray source wavelength (1.54 Å), β is the full width at half maximum (FWHM) of the diffraction peak and θ is the Bragg's angle.
The average particle size of the GO/DETA/MnFe 2 O 4 @SiO 2 composite is obtained based on Debye-Scherrer equation 149 nm.

SEM and EDS analysis
The particle size and surface morphology of MnFe 2 O 4 @SiO 2 and GO/DETA/MnFe 2 O 4 @SiO 2 composite were studied applying SEM technique.As illustrated in Fig. 4a, MnFe 2 O 4 @SiO 2 nanoparticles are mostly in spherical shape and have average particle size about 244 nm. Figure 4b demonstrates the SEM image of GO/DETA/ MnFe 2 O 4 @SiO 2 composite.SEM confirms verify the presence of spherical MnFe 2 O 4 @SiO 2 nanoparticles on the surface of GO/DETA/MnFe 2 O 4 @SiO 2 nanocomposite.The SEM image of GO/DETA/MnFe 2 O 4 @SiO 2 composite obviously illustrates that it has created a layered structure and the spherical MnFe 2 O 4 @SiO 2 nanoparticles have the average particle size about 230 nm.The chemical composition of GO/DETA/MnFe 2 O 4 @SiO 2 composite is studied via EDS analysis.The EDS spectrum of composite is presented in Fig. 5. EDS measurement confirms that the composite contains C, N, O, Si, Mn and Fe.The EDS spectrum of GO/DETA/MnFe 2 O 4 @SiO 2 composite shows the atomic percentage of C, N, O, Si, Mn and Fe are 22.53, 10.71, 45.49, 9.01, 9.52 and 2.74, respectively.

Magnetization analysis (VSM)
The magnetic moment of the MnFe 2 O 4 , MnFe 2 O 4 @SiO 2 and GO/DETA/MnFe 2 O 4 @SiO 2 composite were calculated over a range of applied fields between 10,000 and − 10,000 Oe. Figure 6 shows magnetization curves of the

TGA analysis
Thermo-gravimetric Analysis (TGA) was done to predict the thermal stability of the GO/DETA/MnFe 2 O 4 @ SiO 2 composite.Figure 7 demonstrates the TGA-DTG curve of the GO/DETA/MnFe 2 O 4 @SiO 2 composite.The TGA-DTG revealed three-stage degradation pattern between 25 and 600 °C.The first stage degradation happened between 25 and 135 °C with 11.5%, which could be assigned to the release of adsorbed water in the sample 46 .The second weight loss (18.4%) within 135-250 °C can be related to the degradation of groups that contain oxygen 47 .In the third stage, in the range from 250 to 550 °C, the major weight loss occurred and was decomposed 34.7% of GO/DETA/MnFe 2 O 4 @SiO 2 composite, which can be at attributed to the decomposition of the carbon skeletons of GO and DETA 48 .

Effect of adsorbent dosage
The important factor that influences adsorption processes is adsorbent dosage since it characterizes the capacity of adsorbent for a given initial concentration of dye solution 49 .In our work, the effect of adsorbent dosage on adsorption removal of MB was investigated with sorbent amounts in the range 0-30 mg in the adsorption systems containing 40 mL of 100 mg/l solution of dye at 25 °C for 10 min.Figure 8 displays the effect of adsorbent dosage on the percentage removal of MB.As shown, the percentage removal of MB increased from 0 to 89% with increasing GO/DETA/MnFe 2 O 4 @SiO 2 composite.This phenomenon can be related to the increasing of the surface area of the adsorbent and availability of more adsorption sites.Based on the results in Fig. 8, an optimum adsorbent dosage of 25 mg was selected.www.nature.com/scientificreports/

Effect of initial dye concentration
The influence of initial dye concentration on adsorption in case of GO/DETA/MnFe 2 O 4 @SiO 2 composite was studied with different solution concentrations (30-150 mg/l) applying 25 mg of adsorbent.As it is shown in Fig. 9, the dye removal percentage is decreased with the increase of the initial dye concentration, which may be due to the decrease of enough number of active sites of GO/DETA/MnFe 2 O 4 @SiO 2 composite for binding on the dye molecules.

Effect of contact time
The effect of contact time on the adsorption of MB on GO/DETA/MnFe 2 O 4 @SiO 2 composite is demonstrated in Fig. 10.As can be seen in Fig. 10, with increasing contact time, the adsorption percentage of MB has increased rapidly in the early stages because of high availability of vacant adsorption sites.After a period of 10 min, the adsorption illustrated a steady increase.

Effect of initial pH solution
The pH of solution is an important factor due to separation of different functional groups on the adsorbent and ionization of adsorbent in solution.The effect of pH on the sorption of MB onto GO/DETA/MnFe 2 O 4 @SiO 2 composite was investigated within pH range 2-12 (Fig. 11).As it can be observed in Fig. 11 with increasing the  pH solution, the removal of MB was increased.At higher pH, the surface has a negative charge and produces electrostatic interactions with MB cationic molecules 50 .

Adsorption isotherms
One of the important parameters for study of the interaction between adsorbent and dye are adsorption isotherms 51 .In this study, Langmuir and Freundlich isotherm models were applied to gain the isotherm parameters for adsorption of MB onto GO/DETA/MnFe 2 O 4 @SiO 2 composite.One of the well-known models   where C e is the concentration of the dye solution at equilibrium (mg/l), q e is the maximum amount of dye adsorbed (mg/g), q m indicates the value of monolayer adsorption capacity and K L is the constant value of Langmuir (mg/l).The values of K L and q m were obtained from the plot of (C e /q e ) versus C e .The Langmuir plot for the adsorption of MB onto GO/DETA/MnFe 2 O 4 @SiO 2 composite is illustrated in Fig. 12.
Freundlich isotherm model is used for reversible heterogeneous surface and is showed by the following linearized equation: where K F and n are adsorption capacity (L/mg) and intensity of adsorption, respectively.The K F and 1/n can be calculated from the linear plot of Ln qe versus Ln Ce (Fig. 13).The 1/n values represent irreversible (1/n = 0), favorable (0 < 1/n < 1) or unfavorable (1/n > 1) condition for adsorption.
The separation factor (R L ) was determined by the following equation: where K L is the Langmuir constant and C 0 is the highest initial concentration of adsorbent (mg/l).The values of R L can demonstrate the shape of the isotherm.If R L > 1, the adsorption is unfavorable; if R L = 1, the adsorption is linear; if 0 < R L < 1, the adsorption is favorable and if R L = 0 the adsorption is irreversible.Table 1 shows the values of Langmuir and Freundlich parameters and the regression coefficients (R 2 ) of the MB onto GO/DETA/ MnFe 2 O 4 @SiO 2 composite.Based on R 2 values, the experimental data were found to fit with Langmuir isotherm model for adsorbent.Also, the value of R L lying in the range 0 < R L < 1 confirms the favorable condition for adsorption of MB onto GO/DETA/MnFe 2 O 4 @SiO 2 composite.Based on the result, the Langmuir model suggests (1)

Kinetic model for MB adsorption
For investigating the mechanism of adsorption of MB two kinetic models: (i) pseudo-first order and (ii) pseudosecond order models have been studied.The linear form of pseudo-first-order 52 and pseudo-second-order 53 kinetic are expressed in Eqs. ( 4) and ( 5), respectively: where q e and q t (mg/g) is the amount of dye adsorbed at equilibrium and at time t, K 1 and K 2 (min −1 ) are the rate constants.The adsorption kinetics plots obtained for MB on GO/DETA/MnFe 2 O 4 @SiO 2 composite are demonstrated in Fig. 14.The constants obtained for pseudo-first order and pseudo-second order models are listed in Table 2. Based on the results, the pseudo-first order model fit demonstrates a higher R 2 value compared to the pseudo-second order for adsorbent.Also, the q e value gained by calculating pseudo-first order kinetic is closer to the experimental value (177.25 mg/g).

Thermodynamics of adsorption
In order to study the thermodynamic factors such as standard enthalpy change (ΔH °), standard Gibbs free energy change (ΔG°) and standard entropy change (ΔS °), adsorption studies have been done at different temperatures (285-320 K).The thermodynamic factors were examined using following equations: (4) log q e − q t = logq e − K 1 2.303 t  Table 2. Kinetic parameters for the adsorption of MB onto GO/DETA/MnFe 2 O 4 @SiO 2 composite.

Pseudo-first-order
Pseudo-Second-order q e (mg/g) K 1 (× 10 -3 )(min −1 ) R 2 q e (mg/g) K 2 (× 10 www.nature.com/scientificreports/where K C is the thermodynamic equilibrium constant, T is the solution temperature and R is the universal gas constant (8.314J/mol K).The values of H • and S • were determined from slope and intercept of plot Ln K L vs 1/T (Fig. 15).Table 3 demonstrates the values of various thermodynamic parameters for the adsorption of MB on GO/DETA/MnFe 2 O 4 @SiO 2 composite.The positive value of H • indicates that the adsorption of MB on GO/DETA/MnFe 2 O 4 @SiO 2 composite is endothermic and the positive value of S • suggests the increase in randomness and disorder at the adsorbent-solution interface during the adsorption of MB on GO/DETA/ MnFe 2 O 4 @SiO 2 composite.Also, the negative values of G • show that the adsorption of MB on GO/DETA/ MnFe 2 O 4 @SiO 2 composite is spontaneous process.

Adsorption mechanism
The pH of solution plays an important role because of the separation of different functional groups on the adsorbent and ionization of adsorbent in solution.It was found that the adsorption efficiency increased with increasing pH (Fig. 11).With the increase of pH, the negative charges in the solution are increased.This could be because of the ionization of functional groups such as hydroxyl on the composite.Therefore, increasing the surface charge density raises the MB removal percentage due to the increasing electrostatic interactions between the negative charge of the composite and the positive charge of the MB (Fig. 16) 54 .As a result, the electrostatic interactions between the positive charge on nitrogen group of MB and the negative charge on oxygen group may act as the critical adsorption mechanism 55 .Also, the localized π electrons in the conjugated aromatic rings of the composite can interact by the C-C double bond of MB through π-π interaction 56 .Finally, the hydrogen bonding interactions between amine or oxygen containing groups of composite and hydrogen containing groups of MB play important role in the adsorption of MB on GO/DETA/MnFe 2 O 4 @SiO 2 composite 57 (Fig. 16).

Reusability studies
In the study of adsorption process, reusability is of great importance from the cost point of view in water treatment.To study the regeneration ability of the GO/DETA/MnFe 2 O 4 @SiO 2 composite, four cycles of MB removal were evaluated 58 .The percentage removal of MB in 0.1 M HCl solution is represented in Fig. 17.As it can be observed from Fig. 17, the MB removal percentage decreased slightly and was still 74%.

Comparison with other reported adsorbents
The adsorption capacity of GO/DETA/MnFe 2 O 4 @SiO 2 composite for removal of MB was compared with GO-based adsorbents and other adsorbents reported in the literature (Table 4).As it is shown in Table 4, the adsorption capacity of GO/DETA/MnFe 2 O 4 @SiO 2 composite is acceptable for removal of MB from aqueous solutions.

Conclusion
The GO/DETA/MnFe 2 O 4 @SiO 2 composite has been successfully prepared and characterized by FTIR, XRD, SEM, EDS, VSM and TGA techniques.The composite showed magnetic property with a saturation magnetization of 3 emu/g.It was found that the GO/DETA/MnFe 2 O 4 @SiO 2 composite was effective in adsorption of MB from aqueous solution.The effects of different parameters such as adsorbent dosage, initial drug concentration, pH and  contact time were investigated.The Langmuir isotherm model was the best model to understand the adsorption process.According to the Langmuir analysis, the maximum adsorption capacity (qm) of the adsorbent for MB was obtained to be 243.91 mg/g.Kinetic studies demonstrated that the adsorption process followed pseudo-first order model for MB.The thermodynamic study illustrated that adsorption of MB on composite was spontaneous and endothermic, which was proceeded via hydrogen bonding, electrostatic and π-π interactions.www.nature.com/scientificreports/

Experimental Chemicals and instrumentation
All the chemicals were purchased from Merck.Methylene blue (chemical formula = C 16 H 18 ClN 3 S, Molecular weight (g/mol) = 319.85)was purchased from the Textile Factory.FT-IR spectra (Shimadzu prestige-21) were used to determine the identity of the as prepared nanocomposite.X-ray powder diffraction measurements were performed using an X-ray diffractometer (XRD) (Perkin Elmer) at ambient temperature.The surface morphology of the synthesized compounds was identified with a scanning electron microscope (LECO SEM, Michigan, USA).The elemental analysis was performed using energy-dispersive X-ray spectroscopy (EDS) on a scanning electron microscope, Mira 3-XMU model.Magnetic measurements were performed by means of the vibrating sample magnetometery method, using a VSM 7407 magnetometer, at room temperature.Thermogravimetric analysis (TGA) was performed using a Perkin Elmer thermogravimetric analyzer.UV-visible spectra in the 200-1000 nm range were obtained in DMF solvent on a Perkin Elmer Lambda 45 spectrophotometer.A Jenway model 4510 pH-meter was used for pH measurements by use of a combined electrode.An ultrasonication probe (Karl Deutsch, Germany) was used to disperse the nanoparticles in the solution.

Preparation of the GO/DETA/MnFe 2 O 4 @SiO 2 composite
Preparation of GO GO was prepared using the reported modified method 70 .Graphite powder (2.0 g) were dissolved in a mixture of H 2 SO 4 and H 3 PO 4 (150:15) with stirring at 60 °C.Then, 10 g of KMnO 4 was added to the mixture and stirred for 12 h until its color changed to brown.Afterward, the mixture was cooled then 300 ml deionized water and 4 ml H 2 O 2 (30%) was added.The mixture was centrifuged and the residue was washed with HCl (10%) followed by deionized water several times until the pH achieved neutral and dried under vacuum.

Preparation of Manganese Ferrite Nanoparticles (MnFe 2 O 4 )
MnFe 2 O 4 NPs was prepared using the reported modified method 71 .Briefly, 0.9 g of FeCl MnFe 2 O 4 (6 g) was dispersed in a 100 ml of 0.1 M HCl aqueous solution.Then, solution ultrasonically agitated for 20 min.The nanoparticles isolated and washed with deionized water.Then, the nanoparticles were suspended 40 ml deionized water, 100 ml ethanol and 15 ml 28% ammonia solution and stirred for 1 h.Afterward, tetraethylorthosilicate (20 ml) in 20 ml ethanol was added to solution.This solution was stirred for 10 h at room temperature.Then, the product MnFe 2 O 4 @SiO 2 were separated by an external magnet and washed with deionized water and ethanol.
Preparation of GO/DETA/MnFe 2 O 4 @SiO 2 composite 0.5 g of GO was placed into 100 ml DMF and ultrasonically dispersed for 1 h.Subsequently, 0.5 g MnFe 2 O 4 @SiO 2 and 70 ml Diethylenetriamine (DETA) were added into to flask.The mixture refluxed with stirring for 24 h. the composite was isolated and washed with ethanol and dried in a vacuum oven at 50 °C.

Adsorption experiments
GO/DETA/MnFe 2 O 4 @SiO 2 composite was used for removal of MB dye from aqueous solutions.For this aim, different factors such as adsorbent dose, contact time, initial concentration and PH on adsorption were investigated.For doing the experiments, solution of 100 mg/l of MB was prepared in deionized water.To prepare www.nature.com/scientificreports/ the experimental solutions, different amounts of composite (5-25 mg) were placed in a series of 40 ml of dye solution with different concentrations (40-120 mg/l) in 50 ml glass flasks.To study the effect of contact time of adsorbent on MB, prepared suspension solutions were stirred for 10-20 min.Beside, by using solutions of 0.01 N HCl or NaOH the effect of pH on the amount of adsorption was studied.The concentration of the MB was calculated by UV-spectrophotometer at λ max = 600 nm.Equations ( 8) and ( 9) were used to calculate the amount of dye adsorbed on the adsorbent (qe in mg/g) and the percentage of solution dye removal (R in %): where C 0 and C e are the initial and equilibrium concentration of dye in solution (mg/l), respectively.V is the initial volume of the dye solution (l) and M is the mass of adsorbent used (g).

Table 1 .
Langmuir and Freundlich isotherms parameters and correlation coefficients for the adsorption of MB onto GO/DETA/MnFe 2 O 4 @SiO 2 composite.

(kJ/mol) ΔS ° (kJ/mol) ΔG° (kJ/mol)
3.6H 2 O and 1.5 g ofMnSO 4•H 2 O were dissolved in 200 ml deionized water with stirring at 80 °C.Then, NaOH (8 M) was added slowly to the solution to raise the pH to 10.The solution was stirred under nitrogen gas at 80 °C for 3 h.Afterward, MnFe 2 O 4 NPs precipitates was separated by a magnetic separation and then washed with deionized water and ethanol.Then, the MnFe 2 O 4 NPs were dried at 60 °C for 24 h.

Table 4 .
Comparison of the adsorption capacity of present system with other reported systems.